Hi Jana, Can you try these DRX Flashing (Local Loopback) DRX flashing could be due to local loopback. Check and disable it with:

stty -F /dev/ttyUSB0 -echo

Use minicom to send AT commands:

sudo apt-get install minicom

Open your serial port:

sudo minicom -D /dev/ttyUSB0

Type AT to check response (OK if working). I hope this will work for you and resolve your issue....... ;)